Pyrotechnic compositions containing elemental boron or elemental phosphorus are found in critical components of ammunition and munitions. For example, amorphous elemental boron, combined with strong oxidizers such as potassium nitrate and other additives or binders has been used as a pyrotechnic igniter. Pellets composed of amorphous elemental boron, potassium nitrate, and nitrocellulose are widely used in the ignition train of medium-caliber ammunition because the rather large amount of propellant within the case cannot be ignited evenly and effectively by a primer alone. Other amorphous elemental boron-containing pyrotechnics are known in the art to produce an intense green light upon combustion, which is of use for signaling purposes on the battlefield. In addition, for many years, the U.S. Navy has used a composition containing amorphous elemental boron and barium chromate to provide precise time delays in the functioning sequences of aircraft ejection seats.
In all of these applications, the presence of elemental boron is thought to be critical. Amorphous elemental boron is known in the art as a potent pyrotechnic fuel, typically producing high reaction temperatures and both gaseous and condensed phase products. The particular physical and chemical nature of these products at the temperature they are produced, along with generally rapid combustion rates, is critical to producing the desired effect in the applications described above. For example, the green light often produced by boron-containing pyrotechnics is known to be caused by the presence of transient BO2 radicals in the flame. Other chemicals used to produce green light in pyrotechnics contain copper or barium which have undesirable environmental and toxicological properties.
Despite the widespread utility of amorphous elemental boron in pyrotechnics, this material also has several undesirable characteristics. Among the common pyrotechnic fuels, amorphous boron is one of the most expensive. The method typically used to produce it does not yield a pure material. As a fine powder usually desired for use in pyrotechnics, amorphous boron is not chemically inert at ambient temperatures. It readily oxidizes in moist air. As a result, compositions containing it are notorious for their poor aging characteristics. It is highly preferable for pyrotechnic fuels to be chemically inert at ambient temperatures, and in the presence of moisture. Compositions containing such chemically inert fuels are more likely to remain unchanged over time in storage, thus ensuring a long operable shelf life for the munitions they reside within.
Crystalline elemental boron is known to be quite chemically inert at ambient temperatures, but it is even more expensive than amorphous elemental boron and is not thought to be a practical pyrotechnic fuel. Sabatini, Poret, and Broad noted in Crystalline Boron as a Burn Rate Retardant Toward the Development of Green-Colored Handheld Signal Formulations, Journal of Energetic Materials, 2011, 29, 360-368, that a pyrotechnic composition containing crystalline elemental boron as the only boron-containing fuel could not be ignited, whereas analogues containing amorphous boron were readily ignited.
The use of elemental phosphorus in munitions also has drawbacks. Phosphorus is used to produce thick white smoke for signaling, marking targets, and screening troop movements. Although white phosphorus (WP) offers the greatest visible obscuration performance of any known substance, it presents serious hazards and logistical complications. WP is typically dispersed aerially by bursting mortar or artillery projectiles. Combustion upon contact with the atmosphere produces phosphorus oxides that are extremely hygroscopic. These oxides rapidly absorb atmospheric moisture resulting in the formation of a large and highly effective smoke screen. However, scattered WP particles do not combust instantaneously and therefore can cause collateral damage in combat as well as the contamination of domestic training ranges. Due to the pyrophoric nature of WP, smoke munitions containing this substance must be stored near a source of water so that they may be submerged if damaged.
To address some of the hazards associated with WP, smoke compositions containing red phosphorus (RP) were developed, although this approach has introduced other logistical issues. RP-based smoke compositions slowly degrade in the presence of air and trace moisture, producing acids that corrode munition components and phosphine gas that is highly toxic and flammable. The use of microencapsulated and stabilized RP slows the aging process but does not completely inhibit it. Additionally, RP-based smoke compositions are generally very sensitive to unintended ignition by impact, friction, and electrostatic discharge. Notably, the most problematic characteristics of phosphorus-based smoke munitions are caused by the white or red phosphorus itself, and not the resulting phosphoric acid aerosol produced upon combustion.
Like boron, phosphorus can exist in more crystalline and chemically inert forms. The violet and black allotropes of phosphorus are far less reactive than white or red phosphorus at ambient temperatures. However, practical methods that could be used to mass-produce these phosphorus allotropes have not been reported. And again, as with boron, it is not known whether these unreactive crystalline allotropes could serve as pyrotechnic fuels.
Smoke compositions containing hexachloroethane (HC), zinc oxide, and aluminum were developed in the 1940s. The zinc chloride formed and aerosolized upon combustion is deliquescent making the resulting cloud exceptionally large, dense, and effective for screening. Unlike most phosphorus-based smoke compositions, which burn in direct contact with the atmosphere, HC compositions are usually pressed into steel canisters. The smoke is released through small vent holes at one or both ends. HC smoke grenades are no longer used by the U.S. Army due to the acute toxicity of the smoke, which has caused accidental inhalation-related injuries and deaths. Despite a number of efforts to address this problem, less toxic alternatives with comparable efficacy remain elusive.
Given the problems associated with amorphous elemental boron and elemental phosphorus in pyrotechnics, a need exists for alternatives. It has been discovered that the use of nanostructured crystalline boron phosphide as a pyrotechnic fuel can provide the desirable pyrotechnic qualities of boron and phosphorus, without the problems associated with the use of the elements themselves. The inventive pyrotechnic compositions may be used to produce green light or smoke, with combustion properties that may tailored to suit specific pyrotechnic applications.